The present invention relates to purified substrates and to a method for making same and, more particularly, to substrates such as cadmium telluride, cadmium zinc telluride and cadmium telluride selenide formed from II-VI elements and to the purification thereof.
A current source of cadmium (Cd) and tellurium (Te) is as a by-product of copper mining. Attempt is made by entities in the commercial supply chain to refine the cadmium and tellurium and to remove impurities, such as copper, therefrom before the Cd and/or Te is, for example, provided for use in electronic devices . However, applicants have determined that even the supposedly best and highest quality of commercially available cadmium and tellurium, and compounds like cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe), still often have unacceptable levels of impurities, and especially copper, for use of such materials in certain electronic devices and applications, such as for sensitive infrared detectors. Cadmium telluride selenide (CdTeSe) is also expected to contain similar impurities. Impurities contribute to a degradation of electrical performance as well as to a reduction of expected operational lifetime of the electronic devices.
Typically bulk II-VI materials like CdTe, CdZnTe or CdTeSe will be grown by a well-known horizontal Bridgeman technique or by other well known standard crystal growing processes. Generally this technique and these processes use a seed crystal disposed in a container, such as quartz-ware, which container offers another potential source of contamination by impurities leaching therefrom into the growth.
When such II-VI materials are to be used as a substrate upon which delicate electronic circuits will be formed, it is believed that processing during formation of the electronic circuits causes impurities to flow from the substrate into the desired electronic circuitry, thereby degrading electrical characteristics and performance of the resulting circuits.
An article entitled "Dopant diffusion in HgCdTe grown by photon assisted molecular-beam epitaxy"--T.H. Myers et al, Journal of Vacuum Science Technology, B10(4) , July/August 1992, pgs. 1438-1443, discusses copper as an impurity in layers grown on certain CdTe and CdZnTe substrates. An article entitled "Copper Outdiffusion from CdZnTe Substrates and its Effect on the Properties of MOCVD-Grown HgCdTe"--R. Korenstein et al, Extended Abstracts, 1993 Workshop on the Physics and Chemistry of Mercury Cadmium Telluride and other IR Materials, Oct. 19-21, 1993, pages 53-54, references the above-noted Myers et al article and its discussion of copper diffusion during MBE (sic--PAMBE) growth and/or post growth annealing of epitaxial layers. Neither of these references is deemed to teach or suggest how to purify the underlying substrate for use in creating dependable and reliable electronic circuitry.
Accordingly, it would be desirable to obtain a high quality, substantially impurity-free II-VI bulk material suitable for use in making delicate and sensitive electronic circuitry. It is also desired to have available a process for removing impurities from II-VI commercially available bulk material so that the purified bulk material can be used in conjunction with such circuitry. By "substantially impurity-free" is meant that the level of impurities in the material are below a pre-determined threshold at which concentration operation of the electronic circuitry is not materially adversely affected.